1. Field of the Invention
The present invention relates to an air die type brake press for bending a workpiece to a predetermined angle, and more particularly to a brake press which automatically compensates for springback in the workpiece by determining the overbend required to compensate for springback.
2. Description of the Related Art
In prior art brake presses, the punch penetration into the die to form the selected bend angle is estimated and a test part is bent. The angle of bend is then checked, and punch penetration is reset and the workpiece is bent again. This process is repeated until the correct angle of bend is produced in the workpiece.
The repetitive process of punch penetration into the die to form the selected bend angle in the workpiece is necessitated due to the elastic properties of the workpiece causing the workpiece to springback after being bent.
Significant advantages are available if the trial and error method of determining the proper punch penetration to achieve the selected bend angle can be eliminated. Potential benefits from eliminating this trial and error method include reduced setup time, improved quality control, insensitivity to material property variations, and improved operator safety. It has been estimated that cost savings during set-up alone can approach 18 percent if a suitably reliable method of automatically compensating for springback can be achieved.
Numerous studies have been conducted in an attempt to eliminate the trial and error method of forming angles in workpieces. Typical of these studies is one entitled "An Adaptive Pressbrake Control Using an Elastic-Plastic Material Model," Journal of Engineering for Industry, Volume 104, page 389, November, 1982, by K.A. Stelson and D.C. Gossard. This article was derived from a United States Air Force laboratory test program conducted at the Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, entitled "Sequential Forming of Sheet Metal Parts," Report No. AFWAL-TR-80-4056. The authors designed and built a controller for an air die type brake press which measured force and displacement during the early part of the bend cycle to estimate workpiece parameters. These parameters were then used in an elastic-plastic material model to predict the correct final punch penetration to yield the desired unloaded angle after springback.
The brake press developed as a result of the research under the Air Force test program noted above, included an angle transducer which directly measured the angle of bend between the workpiece and the die of the brake press. The angle transducer was used to determine the difference between the desired angle of bend in the workpiece and the angle of bend of the workpiece after springback. This difference was then added to the initial desired bend angle and the workpiece was rebent to the new angle and the angle of bend after springback was again measured by the angle transducer. This repetitive process was continued until the angle of the workpiece after springback, i.e., the unloaded angle, matched the desired angle of bend.
The report referred to above noted significant drawbacks with respect to determining the unloaded bend angle by means of the angle transducer. The angle transducer consisted of a spring biased pin projecting through a cylindrical hole in the die member of a brake press. One end of the pin contacted the workpiece surface, and the other end was connected to a rotary shaft encoder offset from the die member of the brake press. Using the angle transducer described above to directly measure the angle of springback in a workpiece has significant drawbacks in that when used in an industrial environment a second solenoid is required to withdraw the pin below the surface of the die when loading the workpiece on the brake press. Moreover, each die set would require its own built-in angle transducer which would have to be recalibrated regularly.
Therefore, it is an object of the present invention to provide a means for quickly and accurately determining the cessation of springback motion in a workpiece in order to initiate a sequence of calculations to determine the amount of overbend required to compensate for springback.
It is a further object of the present invention to provide an apparatus and a method for determining the angle of springback in a workpiece, to thereby calculate the amount of overbend required to compensate for springback, which does not require a direct measurement of the angle of bend in the workpiece.
It is still a further object of the present invention to provide a means for determining the amount of overbend required to compensate for springback in a workpiece which can be quickly mounted and dismounted on different brake presses.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.